1. Field of the Invention
This invention relates to image forming apparatuses such as copiers, printers, facsimiles, multifunction apparatuses that print, fax, copy, and so on, and more specifically, to a fixing device which fixes an image formed on a recording medium.
2. Description of the Related Art
Conventionally, as image forming apparatuses such as copiers, printers, facsimiles, multifunction apparatuses that print, fax, copy, and so on, a device which employs a fixing belt being extended and wound around a plurality of rollers as a fixing member is known. Such a fixing device includes a fixing belt formed of an endless belt, a plurality of roller members around which the fixing belt wound for rotatable support, a heater disposed in one of the roller members, and a pressure roller that is a pressure member to contact the fixing belt. The heater heats the fixing belt via the roller member. A toner image formed on a recording medium is conveyed to a nip formed between the fixing belt and the pressure roller, and is fixed onto the recording medium by heat and pressure at the nip.
As an example of the fixing device, an on-demand fixing device which achieves a short warm-up time is disclosed in Japanese Patent Application Publication No. 2007-079040 (JP-2007-079040-A). The on-demand fixing device includes a fixing film (endless film) as a fixing member, a pressure roller as a pressure member, and a heater such as a ceramic heater. The heater is provided inside the fixing film to form a nip by contacting the pressure roller via the fixing film and heat the fixing film. A toner image formed on a recording medium is conveyed to the nip, and is fixed onto the recording medium by heat and pressure at the nip. In this configuration, even when papers of small size are fed continuously and the temperature of a region where the papers are not passing is increased, it is possible to perform paper feed operation continuously without lowering productivity by cooling the region where the papers are not passing using a cooling fan.
Further, Japanese Patent Application Publication No. 2010-066376 (JP-2010-66376-A) discloses a device which includes a first heater which heats a center portion of the recording medium and a second heater which heats end portions of the recording medium. The two heaters are operated independently. When papers of small size are fed continuously, only the first heater is used, and when the papers of large size are fed, the first and the second heaters are used. Thus, it is possible to prevent the temperature of the portion where the papers are not passing from being increased, thereby saving energy.
However, in the fixing device disclosed in JP-2007-79040-A, there is a problem that the device configuration becomes complex and the size of the device increases, and a temperature deviation in the axial direction is likely to occur due to an air flow. In the fixing device disclosed in JP-2010-66376-A, productivity may be lowered when papers of small size such as postcards and envelopes which are smaller than the first heater provided at the center portion, are fed continuously.